Subject:

From: dennis@ngala.as.arizona.edu

Submitted: Fri, 18 Jul 2003 14:15:55 -0700

Message number: 151 (previous: 150, next: 152 up: Index)

Hello All,

In response to Michael's prodding and Dave's "sneak" attack, I've put some various
thoughts on e-paper as an expanded version of Dave's section B (regarding what 
LSST astrometry can do for Galactic Halo studies). Instead of focusing primarily 
on the streams, I would emphasize the structure of the halo (which of course
includes the streams). Many areas need to be fleshed out with real numbers (and
this may be longer than Michael wants already) but perhaps those of us interested
in these topic (Monet, Olsen, Saha, Ivezic, ?) can iterate.

Cheers
-Dennis Zaritsky



B.1 Tracing the Smooth Luminous Halo

From measurements of relatively local stars the current understanding 
appears to be that the galaxy has a stellar halo of
steeply falling density (rho propto r^{-3} Ivezic et al.) and
a possible cutoff at r ~ 50 kpc. Using SDSS data, the halo
and its inhomogeneities, are traced with stars that are brighter than
M ~ 0 at several tens of kpc (m_r ~ 20 at the suggested
cutoff radius). LSST will reach m_r ~ 26,
which is fainter than the main sequence turnoff at the suggested
cutoff radius (M ~ 6). The turnoff stars
then become an excellent tracer of the halo (other types of stars
that are that blue (B- V ~ 0.4) and that faint will be vastly
outnumbered (factor?) by the main sequence stars and should be
available perhaps out to perhaps 100 kpc. Proper motions can be used
to further remove contaminants (foreground white dwarfs will be 
~100 times closer). The tracing of
the halo can be done in a highly statistical manner by matching models
with specific properties to the number counts and proper motion distributions.
One should keep in mind that unlike the current status, where because 
we are relatively data starved we need to be fairly confident of each star
(particularly in the use of giants in the
halo), in the LSST era much more can be done confidently in a 
statistical manner. Even with SDSS, the structures that are being
found are the ones we can point to the overheads (this will change with
time once the cream has been skimmed and the data deluge recedes).

Goals:

1) Confirm/refute presence of cutoff at 50 kpc (trace the 3-D shape
of the cutoff). This is simple for LSST, but see issue about colors
and the use of colors to discriminate giants/dwarfs (below).

2) Identify any further cutoffs (shell structure? multiple shells as
seen in secondary infall calculations?). Tests of smooth infall
vs. hierarchical.

3) Measure radial behavior of stellar halo from 50 kpc to 200+ kpc.

4) Measure velocity ellipsoid vs. r to constrain infall models and
halo orbit families (important for formation/evolution modeling (deposition
of angular momentum) and for mass profile determinations).This would
be done with local halo populations for which the proper motions
would be measurable (the 10-20 km/sec proper motions envisioned for the
final survey would be ideal).

Question: What the relative merits of more colors to do color-color selection
vs. more in one passband to do more precise astrometry?} The clear purpose of
multicolor data is to separate stellar population (i.e. foreground dwarfs from halo
giants, etc.). Some recent results using the SDSS filters suggest that broad band
colors might do a reasonable job. The preferred narrow-band filters are probably
out of the question for LSST. Could they be done (at least in part) by a dedicated
smaller telescope? This topic needs more work on understanding how much can
be gleaned from a relatively standard filter set, and how much cannot.

B.2 Tracing the Unseen Halo

LSST can play a role in the study of Galactic dark matter in at least two 
ways: 1) it will provide dynamical information to map the inner halo via
proper motions, 2) it will identify RR Lyrae stars at large distances to
greatly increase the number of dynamical tracers in the outer halo, 3) 
it will provide the most comprehensive survey of the luminous Galactic
components. 

B.2.1

One of the most compelling arguments for a large dark matter halo comes
from the fastest moving star (one assumes that is is bound and therefore
derives a mass). It is a simple argument. By actually measuring the full
3-D velocities of stars in the local neighborhood, we would have not just
a single fast moving star but a sample of fast moving stars from which 
orbits could be constructed. A self-consistent mass model would not only
need to explain the velocity distribution function but would need to 
tie that into the density of objects at larger radii (i.e. if we find that
many local stars have apogalacticons of 100 kpc, there better be a corresponding
population of stars at 100 kpc). Tracing a population in this manner is 
much more constraining on a model than simply using a satellite galaxy
(or two) at large radii. 

B.2.2 

Dynamical tracers at large radii are important because they provide independent
measures of the total enclosed mass. Again, the consistency required by the
study of these objects with the local velocity distribution function is well
beyond anything available to date. Examining such studies as that by Kochanek (1996)
or Wilkinson and Evans (1999), where the dynamics of various populations at different
radii are combine, the tightening of the constraints by including a range of objects
at different radial scales is evident. The current state of the art includes ~ 30 objects,
ongoing work reaches ~100 objects over 6 Schmidt plates (Clewley et al. 2002). 
LSST will dwarf these studies.

B.2.3

The interpretation of the microlensing results depends on several factors.
The LSST can provide critical information on the radial distribution of 
stellar lensing populations and perhaps some information on the tangential
velocities of lensing sources. The ambiguities in the interpretation that can be introduced by 
deviating from a standard halo can be seen in the study by Geza,
Evans, & Gates (1998). 

Goals:

1) Precise radial profile for stellar halo components.

2) Precise measurement of the structure within the halo (what
are the likely over/under densities along the LMC and SMC lines-of-sight?).

3) Precise measurement of the tangential velocities (perhaps available only for the 
nearer lenses). At 1/2 the distance to the LMC (where lensing is favored), the proper 
motions will be good to 25-50 km/sec and so a lensing populations should be separable from
the LMC stars, and a dispersion may be measurable if it is comparable to that of the
Galactic halo (~ 150 km/sec).

C. Tracing the Lumpy Halo

To best trace the streamers we need both distances and velocities to halo stars. 
LSST provides the best avenues for obtaining each of these over large areas of the sky.
First, RR Lyrae stars will be identified throughout the halo and provide
distances for any overdensity of stars identified as a potential streamer.
MSTO stars can also be used (see above).
Second, proper motion measurements will provide some information on the
kinematic coherence of any overdensity. The First Light observations are
not useful for this program, but the first repeat and then the completed
mission probe the range of expected halo velocities first to about 50 kpc
and eventually to 200 kpc and beyond. Ironically, although radial
velocities are generally much easier to obtain than tangential velocities,
the all-sky nature of LSST will make proper motions easier to obtain
than radial velocities over the same area of sky for the large number
of stars involved. 

With the tremendous number of stars involved, we do not need to identify
individual streamers but rather quantify the ``lumpiness" of the 
velocity distribution to constrain the number of streamers contributing
to the halo population. 

Goals:

1) Completely trace streamers our to several hundred kpc. (do 
streamers come primarily from one type of orbit family, for example,
satellites on radial orbits). 

2) Measure coherence of streamers over full length of arcs (a measurement
of the roughness of the Galactic potential). 

3) Measure width of streamers (a combination of Galactic potential
roughness and the initial internal velocity dispersion of satellite 
- important for understanding initial population). The internal
velocity dispersion of the original object that was disrupted is expected to 
be several tens of km/sec for the most massive objects. It  is therefore
important to obtain proper motions that are as precise as possible and certainly
better than  several tens of km/sec to be able to say anything regarding the
coldness of the stream and potentially about the original system.

4) Measure proper motions along stream to solve for ``guiding center"
orbit (determines potential depth and shape).



Geza, G., Evans, N.W., and Gates, E.I. 1998, ApJL, 502, 29
Iveciz, Z. et al. 2000, AJ, 120, 963
Clewley, L., et al. 200, MNRAS, 337, 87
Kochanek, C.S. 1996, ApJ, 457, 228
Wilkinson, M.I., and Evans, N.W. 1999, MNRAS, 310, 645

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